1. Field of the Invention
The present invention generally relates to a power supply control circuit. More specifically, the present invention relates to a power supply control circuit, electronic apparatus, semiconductor device, control method for a power supply control circuit, and control method for an electronic apparatus, and even more specifically relates to a power supply control circuit, electronic apparatus, semiconductor device, control method for a power supply control circuit, and control method for an electronic apparatus of the type in which electricity generated by a power generation apparatus is stored in a storage device.
2. Background Information
Power supply control circuits provided with an electronic clock that includes a power generation apparatus, wherein the electrical energy generated by the power generation apparatus is stored in a storage device via a so-called blocking diode are conventionally known. For example, Japanese Laid-Open Patent Application No. 9-264971, Japanese Laid-Open Patent Application No. 10-201128, and Japanese Laid-Open Patent Application No. 10-210681 show this type of control circuit.
However, when a solar battery is used as the power supply apparatus, the charging current characteristics of the solar battery include a characteristic whereby a fixed charging current flows until the stored voltage of the storage device reaches a prescribed voltage X(V), and the charging current stops flowing when the stored voltage reaches the prescribed voltage X(V).
In this case, when the configuration includes an ideal power supply control circuit in which a blocking diode is not used, the voltage X is equal to the charging voltage of the storage device. However, in an actual power supply control circuit that uses a blocking diode, the voltage X is equal to the sum of the charging voltage of the storage device and the forward voltage of the blocking diode. Thus, there is a drawback in that charging current of the storage device charges in less than ideal conditions. In other words, the forward voltage of the blocking diode essentially appears as a charge loss.
In the particular case of charging a solar battery under low illumination, since the voltage X is reduced in comparison with high illumination, there is a drawback in that the ratio of charging loss is considerable.
When an attempt is made to install a blocking diode in a semiconductor device (LSI, for example), the impurities in the silicon substrate are commonly diffused to form a p-type diffusion layer and an n-type diffusion layer, and these are structured as a diode by a PN junction. However, since the diode formed by a PN junction on the silicon substrate has a considerable forward voltage, the charging loss increases and the charging efficiency is reduced.
As shown in International publication WO98/21815, technology has been proposed (refer to patent reference 4) to solve the above-described problems, in which a field-effect transistor is connected to a blocking diode in parallel with the blocking diode, electrical energy generated by a power generation apparatus initially charges a storage device by way of the blocking diode, the field-effect transistor is switched ON and the blocking diode is bypassed when the forward voltage of the blocking diode is equal to or greater than a prescribed voltage, and the storage device is charged in a state of low charging loss.
No problems are encountered when a constant-voltage power generation apparatus such as an electromagnetic induction generation apparatus is used for the power generation apparatus to generate a charging current in correspondence with the difference in electrical potential between the power generation voltage of the power generation apparatus and the charging voltage of the storage device. However, in the case that a constant-current power generation apparatus such as a solar battery is used, the blocking diode is shorted by the field-effect transistor and the forward voltage becomes zero when the forward voltage of the blocking diode is detected and the field-effect transistor is switched ON, causing the field-effect transistor to be switched OFF one more time.
When a constant-current power generation apparatus is therefore used as a power generation apparatus, problems are encountered in that the charging loss is not reduced because the storage device is charged as the field-effect transistor repeatedly switches ON and OFF.
When using a configuration in which the field-effect transistor is kept in the ON state for a prescribed length of time, electric current flows back from the storage device to the power supply apparatus when the power generation apparatus no longer produces electricity during that period of time, and a new problem arises in that power is wastefully consumed.
In the present invention, it is possible to enhance battery charge efficiency of an electric timepiece and a portable electric apparatus having a generator or battery and to elongate drivable time, by reducing power loss in charging and power leak, in a power supply circuit.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved power supply control circuit, electronic apparatus, semiconductor device, control method for a power supply control circuit, and control method for an electronic apparatus. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.